(1) Field of the Invention
The present invention generally relates to digital data transmission, and, more particularly, to a technique for transmitting digital data in a noisy environment
(2) Description of the Prior Art
U.S. Pat. No. 4,783,779 to Takahata et al. is directed to frequency assignment in a frequency division multiple access (FDMA) satellite communication system. When a service is a low bit rate signal, the service is transmitted over a single carrier frequency, and its related frequency band. When a service is a high bit rate signal which cannot be transmitted in a single carrier signal, the service is divided into a plurality of low bit rate PSK signals, each of which is transmitted by using a signal carrier frequency and the related frequency band.
However, unlike the satellite environment mentioned above, when digital data is being transmitted via radio on a shared coaxial cable, frequently the data will be contaminated by harmonics or other spurious frequencies that are generated by other signals. Quite often one does not have control of the other signals and so will be unable to reduce the interference.
A digital signal that is used to modulate a carrier by some method such as Biphase Shift Keying (BPSK), Quadriphase Phase Shift Keying (QPSK), amplitude modulation (AM), or frequency modulation (FM) will have a bandwidth that will be proportional to the bit rate of the serial data stream. If the bandwidth of the signal of interest is so wide that the harmonics or spurious signals generated by the other sources contaminate the signal of interest, then it will be necessary to provide some method to overcome this problem.
FIG. 1 shows a typical frequency plot of a spectrum with contaminating harmonic frequencies. The digital data is carried within both the main and side lobes. As shown in FIG. 1, the interfering harmonics 10 (e.g., noisy signals and other transmissions) are present in all the lobes. The carrier is located in the center of the main lobe at position A.
In general, a digital telemetry signal, such as a BPSK, requires a signal to noise ratio (SNR) of about 14 dB to keep the bit error rate at an acceptable level. One solution would be to raise the carrier level of the digital telemetry signal until the SNR was at an acceptable level. However, on a shared coaxial cable the maximum rms voltage level of any signal will be restricted by the other circuit's vulnerability to peak voltage spikes. Thus, there exists a limit to the maximum rms voltage that will be allowed on the coaxial cable.
In addition, noise caused by clock signals transmitted over the same coaxial cable as a signal of interest is particularly problematic in digital communication circuits. The noise from clock signals shows up as phase jitter and causes an unacceptable increase in the bit error rate and SNR.
In light of the foregoing, there exists a need for a method to allow digital telemetry signals to be transmitted on a shared coaxial cable with an acceptable SNR.